The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
In automatic shifting power transmissions, gear ratio changes are effected by selectively connecting members of planetary gear sets. This is accomplished by selectively engaging torque-transmitting devices such as brakes or clutches. For each gear ratio change, there is a corresponding sequence of off-going clutch disengagement and on-coming clutch engagement. By providing a regulated fluid pressure gain to the clutches, the smooth engagement and disengagement of the clutches may be accomplished.
Additionally, a majority of automatic shifting power transmissions employ a hydrodynamic fluid drive, such as a torque converter or a fluid coupling, between the power source (engine) and a multi-speed gear configuration, such as the planetary gear arrangement discussed above. This hydrodynamic fluid drive will allow the vehicle to come to rest without stalling the engine and will provide a measure of isolation preventing the torsional vibrations, caused by the firing events of the engine, from being transmitted though the powertrain.
However, the traditional use of a torque convertor or other hydrodynamic fluid coupling includes several inefficiencies due to the nature of the fluid dynamic coupling and lack of a mechanical coupling between the engine output and the transmission input. Such mechanical couplings provide direct mechanical linkage between the engine output and the transmission input as in a manual transmission employing a driver engaged clutch. However, manual clutches do not provide as comfortable of an operating experience for many drivers.